This invention relates to new analogues of actinomycin D and to a method of preparing them.
Actinomycin D (AMD) is disclosed in German Pat. No. 1,172,680 and is a chromopeptide antibiotic whose potent activity in several tumors, including Wilm's tumor, gestational choriocarcinoma, and Kaposi's sarcoma, has been reported. It has the formula: ##STR1## wherein P is ##STR2## and R is hydrogen. AMD at submicromolar concentrations strongly inhibits DNA-dependent RNA synthesis and, to a lesser extent, DNA synthesis. Its interaction with DNA has been extensively studied, and the details of the mechanism of binding to DNA has been proposed, E. Reich, Cancer Res., 23, 1428 (1963), W. Muller and D. M. Crothers, J. Mol. Biol., 35, 251 (1968), and H. M. Sobell and S. C. Jain, J. Mol. Biol., 68, 21 (1972). It has been assumed that the cytotoxicity of AMD is due to its inhibition of RNA polymerase following the intercalative binding to DNA. It is quite possible, however, that the distortions in helical DNA resulting from the strong noncovalent association with AMD may not be solely responsible for the observed biological effects. For example, Nakazawa et al, J. Org. Chem., 46, 1493 (1981) suggest that an intermediate free-radical form of AMD may be the active form that causes DNA damage and cell death.
Furthermore, the proximal mechanism of biochemical action of AMD, which is evident from the inhibition of RNA synthesis, may not be the principal mechanism of selective cytotoxicity of the agent at the pharmacological level. For it is known that AMD is far more cytotoxic in those proliferating cells in which it inhibits DNA synthesis than in those of liver, kidney, muscle, etc., that are nonproliferating but are heavily dependent upon RNA synthesis for protein renewal.
Another pharmacological behavior of AMD is that it is not metabolized in vivo. Absence of metabolic conversion or detoxification of AMD leads to its accumulation in the cell nuclei of the host organs and causes cumulative toxicity. This acute cumulative toxicity limits the wide clinical application of AMD.
Accordingly, it would be desirable to snythesize new pharmacologically active analogues of AMD having increased drug efficacy. To achieve this, it would be desirable to increase the drug potency, by enhancing drug activity in the tumor cells and decrease toxicity to the host.